<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.js"></script>
</head>
<body>
    <button id="btn">发送验证码</button>
    <script>
        var btn = document.querySelector("#btn")
        var num = 5;
        // btn.onclick = function(){
        //     var timer = setInterval(function(){
        //         btn.innerHTML = num;
        //         if(num>=0){
        //             num--
        //             btn.disabled = true;
        //         }
        //         else{
        //             clearInterval(timer);
        //             btn.disabled = false;
        //             btn.innerHTML = '发送验证码'
        //             num = 5;
        //         }
        //     },1000)

        // }
        
        btn.onclick = function go(){
            setTimeout(function(){
                btn.innerHTML = num;
                if(num>=0){
                    btn.innerHTML = num;
                    btn.disabled = true;
                    go();
                    num--;
                }
                else{
                    btn.disabled = false;
                    btn.innerHTML = '发送验证码'
                    num = 5;
                }
            },1000)
        }
       
      
 
    </script>
</body>
</html>